The present invention is related to methods of enhancing the wettability of composite materials, and in particular to composite materials that may be easily laminated to form components which can withstand thermal cycling.
High temperature superconductors (HTS materials) have increasing utility as power conductors, fault current limiters, and other applications. Since HTS materials are generally brittle oxides, it is common to produce them in composite form, wherein superconducting filaments are supported by a noble metal matrix. The matrix is most commonly silver-based, but any metal or alloy which will withstand superconductor processing conditions and which does not act to poison the superconductor may be used.
It has been observed that in use, superconducting composites often exhibit xe2x80x9cballooning.xe2x80x9d When the composite is immersed in liquid nitrogen or another cryogen suitable for achieving superconducting properties, a certain amount of cryogen collects in interstices between the matrix and the superconducting filaments. When the composite is allowed to return to room temperature, the cryogen expands and causes portions of the composite to deform to form balloons. This deformation subjects the delicate superconducting filaments to fatigue and possibly fracture, degrading or destroying superconductor filaments. A strong need thus exists for a method of preventing ballooning from occurring.
When a superconducting composite is to be laminated by soldering it to another material (e.g., a metal tape which can provide mechanical and/or thermal stabilization), it is important that the laminate be sealed against any infiltration by liquid cryogen to prevent ballooning, and further that it be in good thermal and mechanical contact with the solder in order to avoid the creation of hot spots or stress concentrations. Similarly, when a composite is protectively encapsulated (e.g., with solder or with a polymer), the encapsulating material should be free of defects that might compromise the integrity of the seal. Also, when forming multistrand cables, solder or another material may be used to hold the strands together, and again, a good contact between the solder and the strands should be achieved.
In one aspect, the invention comprises methods of preparing superconducting composites for lamination, encapsulation, or any other process that requires wetting of the composite by a liquid. The method comprises removing a thin layer of matrix material to expose a surface having a low concentration of oxide particles. The layer may be removed, for example, by chemical etching or by electrolytic cleaning. If it is removed by etching, the etchant may be, for example, nitric acid, hydrochloric acid, ferric chloride, ammonia, a caustic solution, or a mixture of these, and may contain other salts such as ammonium bifluoride. The thin layer may be on the order of 0.013 mm thick, or as thick as can be achieved without excessively degrading the properties of the superconductor (e.g., degrading the critical current by more than about 10%). The exposed surface may subsequently be coated with solder, which may wet the surface with a wettability of at least 250 xcexcN/mm, 300 xcexcN/mm, or 350 xcexcN/mm. The solder may be, for example, lead-tin, silver-tin, silver-lead-tin, or lead-indium solder. When the thin layer is removed electrolytically, an electrolyte such as nitric acid, hydrochloric acid, sulfuric acid, sodium hydroxide, ferric chloride, or ferrous chloride may be used. The removal process may leave the exposed surface at least 97% free of foreign particles, at least 99% free, at least 99.5% free, or at least 99.8% free.
By xe2x80x9cnoble metalxe2x80x9d as it is used herein, it is meant any metal whose reaction products are thermodynamically unstable under the reaction conditions employed relative to the desired superconducting ceramic, or which does not react with the superconducting ceramic or its precursors under the conditions of manufacture of the composite. A noble metal matrix may be a metal different from the metallic elements of the superconducting ceramic, such as silver, gold, or their alloys, or it may be a stoichiometric excess of one of the metallic elements of the superconducting ceramic, such as copper.
By xe2x80x9cdesired superconducting ceramicxe2x80x9d as it is used herein, it is meant the oxide superconductor intended for eventual use in the finished article. Typically, the desired oxide superconductor is selected for its superior electrical properties, such as high critical temperature or critical current density. The desired oxide superconductor is typically a member of a superconducting oxide family which has demonstrated superior electrical properties, for example, BSCCO 2223 (including BSCCO (2.1)223) or BSCCO 2212 in the BSCCO family. By xe2x80x9cprecursorxe2x80x9d is meant any material that can be converted to an oxide superconductor upon application of a suitable heat treatment. Precursors may include any combination of elements, metal salts, oxides, suboxides, oxide superconductors which are intermediate to the desired oxide superconductor, or other compounds which, when reacted in the stability field of a desired oxide superconductor, produces that superconductor. For example, there may be included elements, salts, or oxides of copper, yttrium, and barium for the YBCO family of oxide superconductors; elements or oxides of copper, bismuth, strontium, and calcium, and optionally lead, for the BSCCO family of oxide superconductors; elements, salts, or oxides of copper, thallium, calcium and barium or strontium, and optionally, bismuth and lead, for the thallium (TBSCCO) family of oxide superconductors; elements, salts, or oxides of copper, mercury, calcium, barium or strontium, and optionally, bismuth and lead, for the mercury (HBSCCO) family of oxide superconductors. The YBCO family of oxide superconductors is considered to include all oxide superconductors of the type comprising barium, copper, and a rare earth selected from the group consisting of yttrium, lanthanum, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. By xe2x80x9coxide superconductor intermediate to the desired oxide superconductorxe2x80x9d is meant any oxide superconductor which is capable of being converted to the desired oxide superconductor. The formation of an intermediate may be desired in order to take advantage of desirable processing properties, for example, a micaceous structure amenable to texturing, which may not be equally possessed by the desired superconducting oxide. Precursors are included in amounts sufficient to form an oxide superconductor. In some embodiments, the precursor powders may be provided in substantially stoichiometric proportion. In others, there may be a stoichiometric excess or deficiency of any precursor to accommodate the processing conditions used to form the desired superconducting composite. For this purpose, excess or deficiency of a particular precursor is defined by comparison to the ideal cation stoichiometry of the desired oxide superconductor. Thalliation, the addition of doping materials, including but not limited to the optional materials identified above, variations in proportions and such other variations in the precursors of the desired superconducting oxides as are well known in the art, are also within the scope and spirit of the invention.